Ballast circuit for led-based lamp including power factor correction with protective isolation

ABSTRACT

A ballast circuit for a light emitting diode (LED) based lamp including power factor correction with protective isolation. The circuit includes a transformer with electrically isolated windings and a power factor correction circuit that receives no feedback from a secondary winding side of the transformer. An LED-based lamp assembly and a method of driving an LED-based light source are also provided.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a continuation of, and claims priority of,U.S. patent application Ser. No. 12/616,301, filed Nov. 11, 2009 andentitled “BALLAST CIRCUIT FOR LED-BASED LAMP INCLUDING POWER FACTORCORRECTION WITH PROTECTIVE ISOLATION”, now U.S. Pat. No. ______, theentire contents of which are hereby incorporated by reference.

TECHNICAL FIELD

The present application relates to a ballast circuit for a lightemitting diode (LED)-based lamp including power factor correction withprotective isolation.

BACKGROUND

The development of high-brightness LEDs has led to use of such devicesin various lighting fixtures. In general, an LED-based lamp operates ina fundamentally different way than an incandescent, or gas dischargelamp, and therefore may not be connectable to existing lighting fixturesdesigned for those types of lamps. A ballast circuit may be used,however, to allow use of an LED-based lamp as a retro-fit for existinglighting fixtures.

The ballast circuitry for an LED-based lamp generally converts analternating current (AC) input, such as a 120V/60 Hz line input or inputfrom a dimmer switch, to a stable direct current (DC) voltage used fordriving the LED-based lamp. Such circuitry may incorporate a rectifierfor receiving the AC input and a DC-DC converter circuit. The DC-DCconverter circuit may receive an unregulated DC output from therectifier and provide a stable, regulated DC output to the LED-basedlamp.

A variety of DC-DC converter configurations are well-known in the art.Certain types of known DC-DC converter configurations, such as buckconverters, boost converters, buck-boost converters, etc., are generallycategorized as switching regulators. These devices include a switch,e.g. a transistor, which is selectively operated to allow energy to bestored in an energy storage device, e.g. an inductor, and thentransferred to one or more filter capacitors. The filter capacitor(s)provide a relatively smooth DC output voltage to the load and provideessentially continuous energy to the load between energy storage cycles.

One issue with such switching regulator configurations is that there maybe no protective isolation between the unregulated DC voltage and theregulated DC output voltage. In some configurations, the unregulated DCvoltage may be 400 Volts or more. The unregulated DC voltage can bedangerous if inadvertently applied to the load.

To provide protective isolation, therefore, a transformer-basedswitching regulator, such as a known “flyback” converter, may be used.In a transformer-based switching regulator, the primary side of thetransformer may be coupled to the unregulated DC voltage. The regulatedDC output voltage is provided at the secondary side of the transformer,which is electrically isolated from the primary side of the transformer.The transformer may thus provide protective isolation of the DC outputfrom the unregulated DC voltage.

Another issue with switching regulator configurations is that theyinvolve a pulsed current draw from the AC power source in a manner thatresults in less than optimum power factor. The power factor of a systemis defined as the ratio of the real power flowing to the load to theapparent power, and is a number between 0 and 1 (or expressed as apercentage, e.g. 0.5 pf=50% pf). Real power is the actual power drawn bythe load. Apparent power is the product of the current and voltageapplied to the load.

For systems with purely resistive loads, the voltage and currentwaveforms are in phase, changing polarity at the same instant in eachcycle. Such systems have a power factor of 1.0, which is referred to as“unity power factor.” Where reactive loads are present, such as withloads including capacitors, inductors, or transformers, energy storagein the load results in a time difference between the current and voltagewaveforms. This stored energy returns to the source and is not availableto do work at the load. Systems with reactive loads often have less thanunity power factor. A circuit with a low power factor will use highercurrents to transfer a given quantity of real power than a circuit witha high power factor.

To provide improved power factor, some lamp ballast circuitconfigurations are provided with a power factor correction circuit. Thepower factor correction circuit may be used, for example, as acontroller for controlling operation of the transistor switch in a DC-DCconverter configuration such as a “flyback” converter. In such aconfiguration, a power factor controller may monitor the rectified ACvoltage, the current drawn by the load, and the output voltage to theload, and provide an output control signal to the transistor to switchcurrent to the load having a waveform that substantially matches and isin phase with the rectified AC voltage.

SUMMARY

In conventional lamp ballast configurations including atransformer-based switching regulator and power factor controllercircuits, such as those described above, complete isolation between theprimary and secondary sides of the transformer has been sacrificed toprovide a feedback to the power factor controller or, for example, toestablish a common ground path for the circuit. This, however, increasesthe potential risk associated with inadvertent application of theunregulated DC voltage to the load. In addition, when suchconfigurations are used with conventional phase-control dimmingcircuits, the transient response time associated with the powercontroller circuit may not be sufficient to establish acceptable dimmingof the lamp without perceptible flicker.

Consistent with the present disclosure, therefore, there is provided anLED ballast circuit and system that converts AC input such as a 120V/60Hz input into a current source for an LED-based light source. Thecircuit includes complete transformer isolation and may use a singleintegrated circuit power factor controller to produce a pulsating DCoutput current that is amplitude modulated by the power factorcontroller at, for example, 120 Hertz. The resulting input power factormay be set very close to unity. The total harmonic distortion at theinput may be very low, and any conducted EMI may be mitigated by thevariable frequency switching technique. Additionally, the size of thetransformer may be relatively small because of the high frequencyoperation and the switching topology, and the controller bias networkand feedback configuration may eliminate the need for large electrolyticcapacitors, or multiple capacitors, for dimming applications. Thecircuit may thus provide a very high power factor, high efficiency andsmall size that will work with dimmer switches, such as a reverse phaseFET dimmer, without flicker at small conduction angles.

In an embodiment there is provided a ballast circuit to drive a lightemitting diode (LED)-based light source. The ballast circuit includes arectifier circuit configured to receive an AC input voltage and providean unregulated DC voltage, and a transformer. The transformer has aprimary winding coupled to the rectifier circuit, at least one secondarywinding configured to be coupled to the LED-based light source, and afeedback winding, the secondary winding being electrically isolated fromthe primary winding and the feedback winding with no electrical pathbetween the primary winding and the secondary winding or the feedbackwinding and the secondary winding. The ballast circuit also includes aswitch, the switch being configured to close to couple a portion of theunregulated DC voltage across the primary winding and the switch beingconfigured to open to transfer energy from the primary winding to thesecondary winding to provide a DC output voltage to drive the LED-basedlight source. The ballast circuit also includes a power factorcorrection circuit configured to control the switch in response to afirst signal representative of current through the primary winding, asecond signal representative of current through the feedback winding,and a third signal representative of the unregulated DC voltage, with nofeedback signal coupled from the secondary winding to the controller.

In a related embodiment, the ballast circuit may further include aswitched bias circuit, the switched bias circuit including a biascircuit switch configured to close when the switch is open to provide asupply voltage to the power factor correction circuit. In a furtherrelated embodiment, the switched bias circuit may be coupled to thefeedback winding, and the bias circuit switch may be configured to closein response to current through the feedback winding.

In another related embodiment, the ballast circuit may further includean over-voltage protection circuit coupled to the power factorcorrection circuit to disable the power factor controller circuit when avoltage at the transformer exceeds a predetermined level. In a furtherrelated embodiment, the over-voltage protection circuit may include aZener diode coupled to the feedback winding, whereby a breakdown voltageof the Zener diode is exceeded when the voltage at the transformerexceeds the predetermined level thereby disabling the power factorcontroller circuit. In another further related embodiment, theover-voltage protection circuit may include an over-voltage protectionswitch configured to close when the voltage at the transformer exceedsthe predetermined level thereby disabling the power factor controllercircuit.

In still another related embodiment, the power factor correction circuitmay include an integrated circuit power factor controller configured toreceive the first signal, the second signal, and the third signal and toprovide an output to control the switch. In yet another relatedembodiment, the AC input signal may be a 120V AC signal.

In another embodiment, there is provided an LED-based lamp assembly. TheLED-based lamp assembly includes a lamp housing, a light source disposedwithin the lamp housing, and a ballast disposed within the lamp housing.The ballast includes: a rectifier circuit configured to receive an ACinput voltage and provide an unregulated DC voltage; a transformerhaving a primary winding coupled to the rectifier circuit, at least onesecondary winding coupled to the LED-based light source, and a feedbackwinding, the secondary winding being electrically isolated from theprimary winding and the feedback winding with no electrical path betweenthe primary winding and the secondary winding or the feedback windingand the secondary winding; a switch, the switch being configured toclose to couple a portion of the unregulated DC voltage across theprimary winding and the switch being configured to open to transferenergy from the primary winding to the secondary winding to provide a DCoutput voltage to drive the LED-based light source; and a power factorcorrection circuit configured to control the switch in response to afirst signal representative of current through the primary winding, asecond signal representative of current through the feedback winding anda third signal representative of the unregulated DC voltage, with nofeedback signal coupled from the secondary winding to the controller.

In yet another embodiment, there is provided a method of driving anLED-based light source. The method includes: receiving an AC inputsignal, and converting the AC input signal into a regulated DC outputusing a ballast circuit. The ballast circuit includes: a transformerhaving a primary winding, a secondary winding and a feedback winding,the secondary winding being electrically isolated from the primarywinding and the feedback winding with no electrical path between theprimary winding and the secondary winding or the feedback winding andthe secondary winding, and a power factor correction circuit receivingno feedback signal coupled from the secondary winding. The method alsoincludes coupling the regulated DC output to the LED-based light source.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages disclosedherein will be apparent from the following description of particularembodiments disclosed herein, as illustrated in the accompanyingdrawings in which like reference characters refer to the same partsthroughout the different views. The drawings are not necessarily toscale, emphasis instead being placed upon illustrating the principlesdisclosed herein.

FIG. 1 shows a block diagram of a system including an optional dimmercircuit and an LED-based lamp assembly according to embodimentsdisclosed herein.

FIG. 2 illustrates a block diagram of an LED ballast circuit accordingto embodiments disclosed herein.

FIG. 3 is a circuit diagram of an LED ballast circuit according toembodiments disclosed herein.

FIG. 4 is a circuit diagram of another LED ballast circuit according toembodiments disclosed herein.

FIG. 5 is a flow diagram of a method according to embodiments disclosedherein.

DETAILED DESCRIPTION

FIG. 1 is a simplified block diagram of one exemplary embodiment of asystem 100 consistent with the present disclosure. In general, thesystem includes a light emitting diode (LED) ballast circuit 102consistent with the present disclosure for receiving an alternatingcurrent (AC) input AC_(in), either directly or through a known dimmercircuit 104, and providing a regulated direct current (DC) outputDC_(out) for driving an LED-based light source 106. The LED-based lightsource 106 may be a single LED or multiple LEDs interconnected in seriesand/or parallel configurations. In one embodiment, AC_(in) may be aprovided directly from a 120VAC/60 Hz line source. It is to beunderstood, however, that a system consistent with the presentapplication may operate from AC sources, such as a 220-240 VAC at 50-60Hz. In an embodiment including a dimmer circuit 104, the dimmer circuitmay be any known dimmer circuit configuration, such as a reverse phasecontrol dimmer circuit. The configuration and operation of such dimmercircuits are well-known in the art.

The LED ballast circuit 102 may convert the AC input voltage AC_(in) toa regulated DC output voltage DC_(out) with a high power factor, highefficiency, small size and protective isolation. The LED ballast circuit102 and the LED-based light source 106 may thus be provided within asingle lamp housing 108, such as within the housing of a parabolicaluminized reflector (PAR) lamp, to provide a LED-based lamp assembly110 consistent with the present disclosure. The LED-based lamp assembly110 provides a convenient retro-fit for existing lighting fixturesconfigured to energize PAR lamps including non-LED based light sources,e.g. fluorescent or gas-discharge sources. An LED-based lamp assembly110 consistent with the present disclosure may be inserted directly intosuch a lighting fixture to operate on the AC input thereto, and mayoperate with a known dimmer circuit. A lamp including an LED-based lightsource 106 may provide long life and low power consumption compared tothose including non-LED-based light sources.

FIG. 2 is a block diagram that conceptually illustrates thefunctionality of an LED ballast circuit 102 consistent with the presentdisclosure. As shown, an LED ballast circuit 102 consistent with thepresent disclosure may include a rectifier 202, a transformer 204including a primary winding 206, secondary winding 208 and a feedbackwinding 210, a switch 212 for coupling the output of the rectifier 202to the primary winding 206 of the transformer 204, an output stage 214coupled to a secondary winding 208 of the transformer, and a powerfactor controller circuit 216. The ballast circuit may also include anover-voltage protection circuit 218 and/or a switched bias circuit 220.The term “coupled” as used herein refers to any connection, coupling,link or the like by which signals carried by one system element areimparted to the “coupled” element. Such “coupled” devices, or signalsand devices, are not necessarily directly connected to one another andmay be separated by intermediate components or devices that maymanipulate or modify such signals.

In general, the AC input voltage AC_(in) may be coupled to the rectifiercircuit 202, either directly or through a dimmer circuit 104. Therectifier circuit 202 may be configured to rectify AC_(in) to provide anunregulated DC output voltage, i.e. a DC output voltage that followsinstantaneous variations in the AC input voltage. A variety of rectifiercircuit configurations are well-known in the art. In one embodiment, forexample, the rectifier circuit 202 may include a known bridge rectifier.

The output of the rectifier 202 may be coupled to the primary winding206 of the transformer through the switch 212 under the control of thepower factor controller circuit 216. The switch 212 may be a knowntransistor switch, as is commonly used in known switching regulatorconfigurations. In general, when the switch 212 is “closed”, the primarywinding 206 of the transformer 204 is coupled to the output of therectifier 202 and the energy is stored in transformer windings. When theswitch is “open”, the energy stored in the secondary winding 208 iscoupled to the output stage 214. The output stage 214 may include acapacitor that is charged by the energy from the secondary winding 208and discharges through the LED-based light source 106 to drive the lightsource.

The power factor controller circuit 216 may include a known power factorcontroller (not shown) configured to provide an output to the switch 212for controlling the switch 212 in response to a signal representative ofcurrent through the primary winding 206, a second signal representativeof current through the feedback winding 210, and a third signalrepresentative of the unregulated DC voltage, with no feedback signalcoupled from the secondary winding 208 to the controller. The outputfrom the power controller may control the switch so that the current tothe LED-based light source 106 has a waveform that substantially matchesand is in phase with the output of the rectifier 202, thereby providinghigh power factor.

Known power factor controllers useful in an LED ballast configurationconsistent with the present disclosure include known integrated circuitpower factor correction controllers, such as model number L6561 andL6562 controllers presently available from ST Microelectronics ofSunnyvale, Calif. The L6561 and L6563 controllers may, for example, beemployed as a controller in a “flyback” DC-DC converter implementation.Details of this and related alternative applications of the L6561controller are discussed in ST Microelectronics Application Note AN1060,“Flyback Converters with the L6561 PFC Controller,” by C. Adragna and G.Garravarik, January 2003, and ST Microelectronics Application NoteAN1059, “Design Equations of High-Power-Factor Flyback Converters basedon the L6561,” by Claudio Adragna, September 2003, each of which isavailable at http://www.st.com and incorporated herein by reference.Specifically, Application Notes AN1059 and AN1060 discuss one exemplaryconfiguration for an L6561-based flyback converter (High-PF flybackconfiguration) that operates in transition mode and exploits the abilityof the L6561 controller to perform power factor correction, therebyproviding a high power factor single switching stage DC-DC converter.Differences between the L6561 and L6562 controllers are discussed in STMicroelectronics Application Note AN1757, “Switching from the L6561 tothe L6562,” by Luca Salati, April 2004, also available athttp://www.st.com and incorporated herein by reference. For purposes ofthe present disclosure, these two controllers may be discussed as havingsimilar functionality.

In a ballast 102 consistent with the present disclosure, the secondarywinding 208 of the transformer is not electrically coupled in any way tothe primary 206 or feedback winding 210, e.g. there is no common groundelectrical path for the windings and there is no feedback path coupledfrom the secondary winding 208 to the power factor controller circuit216 or any other element on primary winding side of the ballast. Thepower factor controller operates using signals coupled thereto from theoutput of the rectifier 202, and the primary 206 and feedback windings210, but no feedback signal is coupled, e.g. electrically or optically,from the secondary winding 208 to the controller. This provides completeprotective isolation for the high voltages on the primary winding sideof the transformer and the secondary side of the transformer. Inaddition, by not requiring feedback from the secondary winding 208, theoverall size and complexity of the ballast is reduced compared toconfigurations wherein, for example, optically isolated feedback isprovided from the secondary winding 208 to the controller.

As is known, the supply voltage for operating a power factor controllermay be self-supplied in the ballast configuration to ensure a regulated,stable supply to the circuit during operation. In a ballastconfiguration consistent with the present disclosure including theoptional switched bias circuit 220, the switched bias circuit 220 mayestablish a supply voltage to the controller with low transient responsetime and low power dissipation. In the illustrated embodiment, theswitched bias circuit 220 is coupled between the feedback winding 210and the power factor correction circuit 216. In an embodiment includinga L6561 or L6562 power controller, for example, the switched biascircuit may be coupled to the Vcc input of the power factor controllerand may include a transistor switch that turns on when the switch 212 isopened to use energy transferred from the feedback winding 210 forproviding a voltage supply to the power factor controller. Such aconfiguration provides rapid transient response that may be particularlyuseful when the system is implemented with a dimmer circuit 104, such asa phased controlled dimmer circuit.

The optional over-voltage protection circuit 218 may be provided to shutdown or prohibit operation of the power factor controller circuit 216upon the occurrence of an over-voltage condition. For example, if theLED-based load 106 ceases conducting current from the secondary winding208, e.g. if the load is not connected or malfunctions, a dangerousover-voltage condition on the terminals of the transformer 204 mayarise. In the illustrated embodiment, the over-voltage protectioncircuit 218 is coupled between the feedback winding 210 and the powerfactor correction circuit 216. In an embodiment including a L6561 orL6562 power controller for example, the over-voltage protection circuit218 may be coupled to the INV or ZCD input of the power factorcontroller for shutting the controller down if an over-voltage conditionexists.

The optional switched bias circuit 220 and over-voltage protectioncircuit 218 are described herein as being useful in connection with aballast wherein the secondary winding is completely isolated from theprimary and feedback windings and no feedback is coupled from thesecondary winding to the power factor controller. Those of ordinaryskill in the art will recognize, however, that these circuits 218, 220may be provided in a wide variety of ballast configurations. Forexample, these circuits may be included in a ballast configurationincluding different transformer or feedback configuration.

FIG. 3 is a schematic diagram illustrating one exemplary embodiment ofan LED ballast circuit 102 a consistent with the present disclosure. Theillustrated exemplary embodiment includes a rectifier circuit 202 a, atransformer 204 a including a primary winding 206 a, a secondary winding208 a and a feedback winding 210 a, a switch Q2 (212 a) for coupling theoutput of the rectifier circuit 202 a to the primary winding 206 a ofthe transformer 204 a, an output stage 214 a coupled to a secondarywinding 208 a of the transformer 204 a, a power factor controllercircuit 216 a, an over-voltage protection circuit 218 a, and a switchedbias circuit 220 a. The power factor controller circuit 216 a includesan L6561 integrated circuit power factor controller U1, the operation ofwhich is known and described in ST Microelectronics Application NotesAN1060 and AN1059, referred to above. Those of ordinary skill in the artwill recognize, however, that other known power factor controllers maybe used in place of the L6561 controller shown in FIG. 3.

In operation, the AC input to the circuit AC_(in) is coupled to therectifier circuit 202 a, which includes a known bridge rectifier. Therectifier full-wave rectifies the AC input to provide a rectifiedunregulated DC voltage DC_(in). The output of the rectifier DC_(in) isconnected to L1 and C1, which filter noise generated in the circuit.

The primary winding 206 a of the transformer 204 a is coupled betweenthe output of the rectifier circuit 202 a and the drain of Q1 so thatwhen Q1 is conducting, i.e. the switch is closed, current flows from theoutput of the rectifier circuit 202 a through the primary winding 206 ato energize the primary winding 206 a, but when Q1 is not conducting,i.e. the switch is open, essentially no current flows through theprimary winding 206 a. In general, when the switch Q1 is closed, thewindings of the transformer 204 a are energized, and when the switch Q1opens, the polarity of the voltage across the secondary winding 208 aand the feedback winding 210 a reverses to forward bias diodes D3 andD5. When diode D3 is forward biased, energy from the secondary winding208 a charges capacitor C4, which is configured to discharge through theload when the switch Q1 is open.

In general, the power factor controller U1 uses a voltage representativeof the output of the rectifier circuit 202 a (i.e., DC_(in)) as areference to control the level at which the controller U1 switches theswitch Q1 on and off using a gate drive GD output coupled to the gate ofQ1 through R1. This feature allows for a very high power factor ballast.The switching frequency is determined by feedback from the primarywinding 206 a and the feedback winding 210 a.

In particular, a portion of DC_(in) is coupled to the multiplier inputMULT of the controller U1 to provide a signal to the controller U1representative of the unregulated DC voltage DC_(in). The MULT input iscoupled between R2 and the parallel combination of R3 and C5. Selectionof R3 and C5 allows for a tradeoff between ripple and power factorcorrection in the output voltage DC_(out) established by the controllerU1. The source of Q1 is coupled to the current sense CS input of thecontroller U1 and to ground through R6. The current through R6 thusprovides a signal to the controller U1 representative of the currentthrough the primary winding 206 a. The feedback winding 210 a of thetransformer 204 a is coupled through R8 to the zero current detectioninput ZCD of the controller U1 to provide a signal to the controller U1representative of the current through the feedback winding 210 a. Inresponse to the MULT, ZCD and CS inputs, the controller U1 provides avariable frequency gate drive GD output to Q1 for driving the load witha high power factor.

Bias voltage is supplied to the power controller supply voltage inputVcc through R10, which is coupled to Vcc through the switched biascircuit 220 a. When there is no starting pulse at the gate of Q1, nocurrent is provided from the rectifier output to energize thetransformer windings 206 a, 208 a, 210 a. Once the voltage on Vccreaches its minimum value, the gate drive output GD of the controller U1provides a starting pulse to the gate of Q1 through R1 to close theswitch Q1 so that at least a portion of the rectifier output is providedacross the primary winding 206 a to energize the transformer windings206 a, 208 a, 210 a.

The drain current in Q1 begins to ramp up at a rate determined by theprimary inductances of the transformer 204 a. This current produces avoltage across R6, which is representative of the current through theprimary winding 206 a. This current is fed into the current sense CSinput of the controller U1. The controller U1 compares this voltage tothe voltage on the multiplier input MULT and the voltage on invertinginput INV, which is set by R2, R3, and the parallel combination of R4and C3 coupled in series with R5. When the voltage conditions are metaccording to the switching characteristics set by the controller U1, thedrive to Q1 is removed. This causes the voltage across the primarywinding 206 a and the secondary winding 208 a of the transformer 204 ato reverse. The energy stored in the transformer 204 a is thentransferred to the output via D3. During this same time interval, thetransformer 204 a provides a voltage on the feedback winding 210 a thatforward biases D5 to provide current to the switched bias circuit 220 a.

In the illustrated embodiment, the switched bias circuit 220 a includesbias circuit switch Q2, R7, R9, and Zener diode D6. R9 and D6 arecoupled to the gate of Q2, R7 is coupled to the source of Q2, and thedrain of Q2 is coupled to R10, C6, and the Vcc input of the controllerU1. D5 is coupled to R9 and R7. When D5 is forward biased, a current isestablished through R9 that turns Q2 on once the gate signal reaches thethreshold voltage of Q2. Q2 charges C6, which provides supply voltage tothe Vcc input. Q2 switches on quickly to provide supply voltage to Vccwith low power dissipation. After all the energy is removed from thetransformer, the voltage on the feedback winding drops to zero. Thisnegative transition on the zero current detection input ZCD of thecontroller U1 instructs it to start a new cycle. After several cycles,the bias voltage on Vcc reaches its normal operating level determined bythe Zener diode D6.

The over-voltage protection circuit 218 a in the illustrated embodimentincludes Zener diode D7. When D5 is forward biased by the feedbackwinding 210 a, if the voltage across the feedback winding 210 a exceedsa predetermined acceptable level, the breakdown voltage of D7 isexceeded and voltages are established at the inverting input INV andCOMP input by R5, R4, and C3 that will shut down the controller U1 toopen Q1. The over-voltage protection circuit 218 a thus disables thecurrent supply to the transformer 204 a to provide protection againstdangerous voltages occurring in the circuit due, for example, todisconnection or malfunction of the load.

FIG. 4 is a schematic diagram illustrating another exemplary embodiment102 b of an LED ballast circuit consistent with the present disclosure.The embodiment illustrated in FIG. 4 is configured and operates inessentially the same manner as described above with respect to FIG. 3,with the main differences being in the configuration and operation ofthe over-voltage protection circuit, and a further difference being thatthe controller in FIG. 4 is a L6562 controller.

The over-voltage protection circuit 218 b in FIG. 4 includes Zener diodeD7, R5, over-voltage protection circuit switch Q3 and R11. The collectorof Q3 is coupled to the zero current detection input ZCD of thecontroller. When D5 is forward biased by the feedback winding, if thevoltage across the feedback winding exceeds a predetermined acceptablelevel, the breakdown voltage of D7 is exceeded and a voltage isestablished across R11 at the base of Q3 that turns Q3 on. When Q3 is ona current is established through R8 to provide a voltage at the ZCDinput that will shut down the controller to open Q1. The over-voltageprotection circuit 218 b thus disables the current supply to thetransformer to provide protection against dangerous voltages occurringin the circuit due, for example, to disconnection or malfunction of theload.

A ballast circuit consistent with the present disclosure may beconfigured for operation with a variety of input voltages based onappropriate selection of various circuit components thereof. Table 1below identifies one example of circuit components useful in configuringthe embodiment illustrated in FIG. 4 for operation with a 120V RMs/60 HzAC input signal (resistor values in ohms):

TABLE 1 Component Descriptor/Value ACin 120 VAC/60 Hz C1 200 nf C2 200nF C3  1 nF C4  10 uf C5  1 nF C6  10 uF D1  1 A D3  1 A D4 220 V D5BAS16 D6  15 V D7  35 V Dout  27 V DC L1 222 uH Q1 TK4P60 Q2 BSS131 Q32N2222 R1  10 R2  1M R3  6.8k R4 180k R5  98.9k R6  2 R7  10 R8  47k R9100k R10 110k R11  5K T1  22 mm EI core LP = 1.5 mH

FIG. 5 is a block flow diagram of a method 500 for driving an LED-basedlight source consistent with the present disclosure. The illustratedblock flow diagram may be shown and described as including a particularsequence of steps. It is to be understood, however, that the sequence ofsteps merely provides an example of how the general functionalitydescribed herein may be implemented. The steps do not have to beexecuted in the order presented unless otherwise indicated.

In the exemplary embodiment illustrated in FIG. 5, an AC input signal isreceived 502. The AC input signal is converted 504 into a regulated DCoutput using a ballast circuit including a transformer having a primarywinding, a secondary winding and a feedback winding, the secondarywinding being electrically isolated from the primary winding and thefeedback winding with no electrical path between the primary winding andthe secondary winding or the feedback winding and the secondary winding,and a power factor correction circuit receiving no feedback signalcoupled from the secondary winding. The DC output is coupled 506 to theLED-based light source to drive the light source.

Unless otherwise stated, use of the word “substantially” may beconstrued to include a precise relationship, condition, arrangement,orientation, and/or other characteristic, and deviations thereof asunderstood by one of ordinary skill in the art, to the extent that suchdeviations do not materially affect the disclosed methods and systems.

Throughout the entirety of the present disclosure, use of the articles“a” or “an” to modify a noun may be understood to be used forconvenience and to include one, or more than one, of the modified noun,unless otherwise specifically stated.

Elements, components, modules, and/or parts thereof that are describedand/or otherwise portrayed through the figures to communicate with, beassociated with, and/or be based on, something else, may be understoodto so communicate, be associated with, and or be based on in a directand/or indirect manner, unless otherwise stipulated herein.

Although the methods and systems have been described relative to aspecific embodiment thereof, they are not so limited. Obviously manymodifications and variations may become apparent in light of the aboveteachings. Many additional changes in the details, materials, andarrangement of parts, herein described and illustrated, may be made bythose skilled in the art.

What is claimed is:
 1. A method of driving an LED-based light source,the method comprising: receiving an AC input signal; converting the ACinput signal into a regulated DC output using a ballast circuitcomprising: a transformer having a primary winding, a secondary windingand a feedback winding, the secondary winding being electricallyisolated from the primary winding and the feedback winding with noelectrical path between the primary winding and the secondary winding orthe feedback winding and the secondary winding, and a power factorcorrection circuit receiving no feedback signal coupled from thesecondary winding; and coupling the regulated DC output to the LED-basedlight source.
 2. A method according to claim 1, wherein receiving an ACinput signal comprises receiving the AC input signal from a dimmercircuit.
 3. A method according to claim 1, wherein converting comprises:converting the AC input signal into a regulated DC output using aballast circuit comprising: a transformer having a primary winding, asecondary winding and a feedback winding, the secondary winding beingelectrically isolated from the primary winding and the feedback windingwith no electrical path between the primary winding and the secondarywinding or the feedback winding and the secondary winding, and a powerfactor correction circuit receiving no feedback signal coupled from thesecondary winding; and a switched bias circuit including a bias circuitswitch configured to close to provide a supply voltage to the powerfactor correction circuit.
 4. A method according to claim 1, whereinconverting comprises: converting the AC input signal into a regulated DCoutput using a ballast circuit comprising: a transformer having aprimary winding, a secondary winding and a feedback winding, thesecondary winding being electrically isolated from the primary windingand the feedback winding with no electrical path between the primarywinding and the secondary winding or the feedback winding and thesecondary winding, and a power factor correction circuit receiving nofeedback signal coupled from the secondary winding; and an over-voltageprotection circuit coupled to the power factor correction circuit todisable the power factor controller circuit when a voltage at thetransformer exceeds a predetermined level.